Evaluating the carbon depletion found by the Stardust mission in Comet 81P/Wild 2

Abstract

The low abundance of refractory carbonaceous material in samples collected by Stardust in comet 81P/Wild 2 coma was completely unexpected. If these results are universal to other comets, this necessitates a reformulation of current models of solar system formation. A polarimetric imaging analysis demonstrates that dust is not uniformly distributed within cometary coma, and that the circumnucleus halo region where the dust samples were collected must contain a low population of carbonaceous particles. Such regions are seen in other comets, suggesting that comet 81P/Wild 2 is not unusual and that the anomalous lack of carbon is not necessarily representative of the entire coma.

Linear polarization measurements of Comet C/2011 W3 (Lovejoy) from STEREO

“…The Stardust mission saw an unexpectedly low abundance of refractory carbonaceous material in samples collected at its closest approach to the nucleus of Comet 81P/Wild 2 (Ishii et al, 2008). Zubko et al (2012) relates this to the strong negative polarization of up to -6% seen in the circumnucleus halo of some comets, including Comet 81P/Wild 2. Their modeling predicts negative polarization as high as -13% for non-absorbing particles with (monodisperse) size parameter x = 10, which they identify with magnesium-rich silicates (though they also point out that the negative polarization would be lower for a polydisperse size distribution).…”

“…Kimura et al 2003;Moreno et al 2007;Das et al 2008), and with the discrete dipole approximation (e.g. Lumme et al 1997;Moreno et al 2007;Shen et al 2009;Zubko et al 2012Zubko et al , 2013.…”

The Composition of Comets

“…To interpret the slightly bluer color in the visible and near infrared in the dust ejecta after the Deep Impact event, small submicron-sized grains were suggested, as well as the presence of pure or mixed water ice crystals or the sublimation of organics (Hodapp et al 2007;Fernández et al 2007;Beer et al 2009). Generally, bluer color requires relatively transparent grain materials such as water ice, Mg-rich silicates, or perhaps non-radiation damaged nor heated organics (Kiselev et al 2004;Zubko et al 2011Zubko et al , 2012Hadamcik et al 2014).…”

“…Compact porous particles also dominated the coma of long period comet C/2007 N3 (Lulin), as evidenced by a negative branch that extended to near-IR wavelengths (Woodward et al 2011). Finally, whenever polarimetric phase curves are obtained on a large enough range of phase angles in different wavelengths (as possible mostly for Halley and Hale-Bopp), numerical and experimental simulations may be used to infer some average properties, such as size and size distribution, ratio between transparent and absorbing materials, and ratio between fluffy aggregates and compact particles (Levasseur-Regourd et al 2008;Lasue et al 2009;Zubko et al 2012;Hines et al 2014). The circumnucleus polarimetric halo provides an example of the complexity of interpretations.…”

Imaging polarimetry of comet 73P/Schwassmann–Wachmann 3 main fragments during its 2006 apparition

“…Different interpretations have been suggested, difference in the size distribution in the inner coma or higher number density of the particles inferring multiple scattering, presence of relatively transparent particles (silicates, low absorption organics, ices; see e.g. Hadamcik et al, 2011;Zubko et al, 2012Zubko et al, , 2013. Jewitt (2004) interpreted the increasing polarization with increasing nucleus distance by a fading of the particles (fragmentation and sublimation of ices).…”